Experimental Investigation of Core Crystallization in Small Terrestrial Bodies
Abstract
Core crystallization is a crucial ingredient in the evolution of terrestrial bodies and is controlled primarily by chemistry and temperature. Transport properties, such as electrical resistivity, are a relevant probe of core crystallization processes, as variations in mass and heat transport in the cooling fluid likely impact the convective and diffusive mechanisms that govern the structure and dynamics of the core and might contribute to generate a magnetic field.
Electrical experiments are reported on core analogues in the Fe-S, Fe-S-Si, and Fe-Si systems from 3.2 to 8 GPa and up to 1850◦C using the multi-anvil apparatus. Electrical resistivity was measured using the four-electrode method. For all samples, resistivity increases with increasing temperature. In the Fe-S system, the higher the S content, the higher the resistivity and the resistivity increase upon melting. The resistivity of FeS and FeSi2 at 4.5 GPa is comparable at temperature below the melting temperature, whereas FeS becomes more resistive than FeSi2 by a factor of two upon melting, suggesting a stronger influence of S than Si on liquid resistivity. Electrical results are used to develop crystallization-resistivity paths. For instance, at 4.5 GPa, equilibrium crystallization, as expected locally in thin snow zones during top-down core crystallization, presents electrical resistivity variations from about 300 to 190 microhm-cm for a core analogue made of Fe-5 wt.%S, depending on temperature. Fractional crystallization, which is relevant to core-scale cooling, leads to more important electrical resistivity variations in the Fe-S system, depending on S distribution across the core, temperature, and pressure. Estimates of the lower bound of thermal resistivity are calculated using the Wiedemann-Franz law. Comparison with previous works indicates that the thermal conductivity of a metallic core in small terrestrial bodies is more sensitive to the abundance of alloying agents than that of the Earth's core. Application to Ganymede and Mercury using core adiabat estimates from previous studies will be presented. This experimental investigation underlines the importance of crystallization-induced distribution of alloying agents across the core on the transport properties of cooling terrestrial bodies.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMMR42A..08P
- Keywords:
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- 3909 Elasticity and anelasticity;
- MINERAL PHYSICSDE: 3919 Equations of state;
- MINERAL PHYSICS